Low rpm loaded but good rpm unloaded at wot

[rbsangus]

Hi all,

Looking for a little assistance on possible issues with a Yanmar 6LPA-STP in a 2002 28TE. In a nutshell, the engine is stabilizing around 3400 (cross checked with strobe) at full power loaded. Boat makes about 20.5 knots at WOT. Form what I can tell that is probably an accurate speed for 3400 rpm.

Book on the motor says 3800 should be max loaded at WOT. To answer a few questions up front:

-the throttle cable and throw does allow for movement all the way to the max stops (although governor lever is about 1/4 inch from stops when reaching 3400.....last 1/4 inch of movement on governor lever creates no change inrpm).
-engine makes book numbers at WOT unloaded (stabilizes around 4100.,,,again cross checked with strobe)
-bottom is clean
-engine just had brand new turbo professionally installed due to black smoke and extremely low rpm.
-no black or white smoke at WOT loaded or unloaded (a little stem above 3000 but nothing excessive).
-high speed stop bolt on governor is still wired from factory

Anyway, if you've read this far, thanks. From my basic knowledge and a little reading I'm looking at 2 possibilities:

-boat is over propped (i.e. Governor is commanding max fuel but reaching less than book WOT loaded numbers due to load)
-boat isn't getting enough gas (i.e. Fuel solenoid isn't fully open or maybe air getting into fuel line)

Any input would be appreciated before I start turning a wrench and wasting time chasing non-issues.

Thanks in advance,

Roy

[Jeremyvmd]

I would look at your prop first that's IMO the likely reason

[Tree]

Running the same engine and probably the same ratio transmission as yourself I see 21knots at 3850 rpm swinging a 19 x 19 x 5 cup propeller. Swinging a 20.5 x 19.5 x 3 cup will see me about 23-24 knots depending on laden weight. Have you suddenly lost rpm or has it always been like this?

[rbsangus]

Thanks. To be honest I don't know if it's always been that way.....just got the boat. Survey was reading low rpm at wot but it was explained as a throttle cable issue. Now know it's not that.

Will keep the power back a bit till we pull it for the season then get a good look at what prop is on it.

[RobS]

......Survey was reading low rpm at wot but it was explained as a throttle cable issue. Now know it's not that.

Explained? By who?
And why "explained" and not "investigated and confirmed" ?

[Norseman]

Thanks. To be honest I don't know if it's always been that way.....just got the boat. Survey was reading low rpm at wot but it was explained as a throttle cable issue. Now know it's not that.

Will keep the power back a bit till we pull it for the season then get a good look at what prop is on it.

Well, what did you find out?

[rbsangus]

Update...was 100% prop issue. Not sure why but the previous owner had a 21 inch prop on the boat with a big pitch on the blades. Got a new 19x19 put on and boat hits all the numbers now.